1. Field of the Invention
The present invention relates to an electronic ballast for controlling fluorescent or high-intensity discharge lamps, and more particularly, to an electronic ballast that requires fewer internal and external components.
2. Description of the Related Art
Electronic ballasts for controlling fluorescent or high-intensity discharge (HID) lamps usually require electronics necessary for preheating the lamp filaments, striking the lamp, driving the lamp to a given power, detecting lamp fault conditions, and safely deactivating the circuit.
Electronic ballasts for gas discharge circuits have recently come into widespread use because of the availability of power MOSFET switching devices and insulated gate bipolar transistors (xe2x80x9cIGBTsxe2x80x9d) that can replace previously used power bipolar switching devices. Monolithic gate driver circuits, such as the IR2155 sold by International Rectifier Corporation and described in U.S. Pat. No. 5,545,955, the disclosure of which is incorporated herein by reference in its entirety, have been devised for driving the power MOSFETs or IGBTs in electronic ballasts.
The IR2155 gate driver IC offers significant advantages over prior circuits: The driver is packaged in a conventional DIP or SOIC package. The package contains internal level shifting circuitry, under voltage lockout circuitry, deadtime delay circuitry, and additional logic circuitry and inputs so that the driver can self-oscillate at a frequency determined by external resistors RT and capacitors CT.
Although the IR2155 offers a vast improvement over prior ballast control circuits, it lacks a number of desirable features such as the following: (i) a start-up procedure which ensures a flash-free start without an initial high voltage pulse across the lamp, (ii) non-zero voltage switching protection circuitry, (iii) over-temperature shutdown circuitry, (iv) DC bus and AC on/off control circuitry, and (v) near or below resonance detection circuitry.
U.S. Pat. No. 6,211,623 to Wilhelm et al. issued Apr. 3, 2001 and having common assignment with the present application discloses an electronic ballast which addresses limitations of the IR2155, such as those discussed above. The electronic ballast is identified by the assignee, International Rectifier Corporation, as the IR2157.
The ballast control circuit of the ""623 patent, as in commonly known ballasts, requires an implementation of the preheat timer that includes a comparator for comparing the CPH pin against a fixed threshold. In addition, the oscillator circuit requires more than one comparator. These and other configuration details result in additional components being required both inside and outside the chip. Accordingly, the prior art could be improved upon by providing a ballast control IC which performs the primary ballast functions while minimizing the internal and external component count. Applications for such ballasts would include linear fluorescent lamps, compact fluorescent lamps (CFL), cold-cathode fluorescent lamps (CCFL), high-intensity discharge (HID) lamps, and flat fluorescent lamps.
The present invention overcomes the deficiencies of the prior art, such as those described above, by providing an electronic ballast that utilizes fewer comparators and combines the functionality of sub-circuits, thereby reducing the number of internal and external components required.
More specifically, the chip of the present invention includes an oscillator circuit that advantageously requires only one comparator circuit. In addition, a lamp preheat circuit uses a preheat resistor parallel to the timing resistor to program preheat frequency, and the voltage at the gate of a MOSFET switch is ramped to gradually disconnect the preheat pin (and thus the preheat resistor) from the frequency timing input. Also, the preheat circuit ramp is utilized as a ramp for lamp ignition as well, thus saving on circuit components. Further, the preheat capacitor input is used as a convenient delay for connecting a DC bus sensing resistor to a DC bus sensing input.
The integrated circuit of the present invention includes circuitry for performing the following functions: micro-power start-up current, programmable preheat frequency, programmable ignition-current or over-current, programmable preheat time, programmable ignition ramp, programmable running or minimum frequency, programmable dead-time, programmable low DC bus frequency-shift reset, external shutdown pin, and high-and low-side 600 V half-bridge driver outputs for driving two MOSFETs or IGBTs connected in a classic totem-pole configuration.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.